1. Field of the Invention
The present invention generally relates to photovoltaic cells and, more particularly, to an improved multijunction photovoltaic cell having a thin top subcell and a thick lower subcell whose energy bandgaps are the same or close to one another which results in improved current matching, adaptability, radiation resistance, and higher voltages.
2. Description of Related Art
The interest in photovoltaic (PV) cells continues as concerns over pollution and limited resources continue. The continued interest has been in both terrestrial and non-terrestrial applications. In the non-terrestrial environment of outer space, the concern over limited resources of any type is a major one. This is because the need to increase the amount of a resource increases the payload. An increased payload can increase the cost of a launch more than linearly. With the ready availability of solar energy in outer space for a spacecraft such as a satellite, the conversion of solar energy into electrical energy is an obvious alternative to increased payload.
The cost per watt of electrical power generation capacity of photovoltaic systems is the main factor which inhibits their widespread use in terrestrial applications. Conversion efficiency of sunlight to electricity is of critical importance for terrestrial photovoltaic systems, since increased efficiency means that all area-related components of the electricity generation system, i.e., cell area, module or collector area, support structures, land area, etc., can be reduced when the efficiency is increased, for a given required power output of the system. For example, in concentrator photovoltaic systems which concentrate the sunlight, typically from 2 to 2000 times, onto the photovoltaic cell, an increase in efficiency means that the area of the expensive concentrating optics can be reduced proportionally. Concentrator photovoltaic systems are a likely application of the multijunction cells described in this invention, because the cell area can be reduced by the concentration ratio, allowing the use of relatively complicated solar cells with a high cost per unit area.
Irrespective of the application, and as with any energy generation system, efforts have been ongoing to increase the output and/or efficiency of PV cells. In terms of output, multiple cells or layers having different energy bandgaps have been stacked so that each cell or layer can absorb a different part of the wide energy distribution of photons in sunlight. The stacked arrangement has been provided in a monolithic structure on a single substrate or on multiple substrates. Examples of multi-cell devices are shown in Kurtz et al., "Modeling of two-junction, series-connected tandem solar cells using top-cell thickness as an adjustable parameter," J. Appl. Phys. 68(4), pp. 1890-1895, Aug. 15, 1990; and U.S. Pat. Nos. 5,800,630; 5,407,491; 5,100,478; 4,332,974; 4,225,211; and 4,017,332.
In the multiple cell device, semiconductive materials are typically lattice-matched to form multiple p-n (or n-p) junctions. The p-n (or n-p) junctions can be of the homojunction or heterojunction type. When solar energy is received at a junction, minority carriers (i.e., electrons and holes) are generated in the conduction and valence bands of the semiconductor materials adjacent the junction. A voltage is thereby created across the junction and a current can be utilized therefrom. As the solar energy passes to the next junction which has been optimized to a lower energy range, additional solar energy at this lower energy range can be converted into a useful current. With a greater number of junctions, there can be greater conversion efficiency and increased output voltage.
Whether in the multiple-junction or single-junction PV device, a conventional characteristic of PV cells has been the use of a single window layer on an emitter layer disposed on a base/substrate, which is shown for example in U.S. Pat. No. 5,322,573. Similarly, a single layer back-surface field structure below the base/substrate has been used, as shown in U.S. Pat. No. 5,800,630. The purpose of the back-surface field structure has been to serve as a passivation layer, like the single window layer described above.
For the multiple-cell PV device, efficiency is limited by the requirement of low resistance interfaces between the individual cells to enable the generated current to flow from one cell to the next. Accordingly, in a monolithic structure, tunnel junctions have been used to minimize the blockage of current flow. In a multiple wafer structure, front and back metallization grids with low coverage fraction and transparent conductors have been used for low resistance connectivity.
Another limitation to the multiple cell PV device is that current output at each junction must be the same for optimum efficiency in the series-connected configuration. Also, there is a practical limit on the number of junctions, since each successive junction generates a smaller current.
The performance limitations above can be found, for example, in PV devices that include GaInP/GaAs, GaInP/GaAs/Ge, GaInP/GaAs/GaInNAs/Ge, and GaInP/Ge. The above structures have relatively high current densities, which can pose problems for current matching subcells that are poor current producers. The structures also have comparatively low voltages, resulting in greater power loss due to series resistance. Subcell base thicknesses can be large, and some subcells have little excess photogenerated current density, both of which impair radiation resistance.
To increase efficiency in a two junction cell, Kurtz et al., supra, have reduced the thickness of the top cell. According to Kurtz et al., the primary increase in efficiency results from an increase in current as the top cell thickness is optimized. The materials systems mentioned are Ga.sub.0.5 In.sub.0.5 P/GaAs, InP/Ga.sub.0.5 In.sub.0.5 As, and GaAs/Ge dual-junction cells. Contour plots are shown for a range of bottom cell bandgaps from .about.0.6 to 1.5 eV, combined with top cell bandgaps from .about.1.35 to 2.1 eV, with emphasis on the ranges of 1.1 to 1.5 eV for the bottom cell combined with a 1.7 to 2.1 eV top cell. Dual-junction cells with the same bandgap or the same semiconductor material for the top and bottom cells are not mentioned in the text of the article. The emphasis of the paper is on GaAs/Ga.sub.0.5 In.sub.0.5 P cells.
What does not appear to be taught by Kurtz et al., among other things, are multijunction cells having more than two junctions, and multijunction cells in which a single subcell has been replaced by two subcells of the same semiconductor material interconnected by a tunnel junction, such as Ga.sub.0.5 In.sub.0.5 P/Ga.sub.0.5 In.sub.0.5 P and Ga.sub.0.5 In.sub.0.5 P/Ga.sub.0.5 In.sub.0.5 P/GaAs multijunction cells. Also omitted is an apparent discussion of how replacing a single subcell with two subcells of the same semiconductor material can: 1) make it possible to current match to 3.sup.rd, 4.sup.th, . . . nth subcells in a series-interconnected multijunction cell, by replacing the current density of the multijunction cell at its maximum power point; 2) essentially double the voltage contribution of the subcell that was replaced by two subcells of the same semiconductor material, thereby reducing series resistance losses and reducing the number of cells that must be interconnected in array strings to reach a given array voltage; 3) increase resistance to radiation damage that is encountered by photovoltaic cells, due to the reduced subcell base thickness that can result when replacing a single subcell by two subcells of the same semiconductor material, as well as to the opportunity for giving greater excess photogenerated current density to other subcells in the multijunction stack because of the reduced current density of the multijunction cell, and 4) increase multijunction cell efficiency.
As can be seen, there is a need for an improved multijunction photovoltaic cell that has increased efficiency and higher radiation resistance. Also needed is a multijunction photovoltaic cell that provides higher voltage, thus reducing series resistance in the cell. Another need is for a photovoltaic cell that has current matched subcell, even when a subcell is of poor material quality, is thinner that optimal, and has a wider bandgap than optimal.